This invention relates to austenitic low expansion alloys, and more particularly to austenitic nickel-iron or nickel-iron-cobalt alloys having in cast form a low thermal expansion coefficient together with good tensile strength at service temperatures.
It is well known that certain austenitic alloys of nickel and iron possess unusual thermal expansion characteristics. For example, an austenitic alloy of 36% nickel and 64% iron has a coefficient of thermal expansion approaching zero between the temperatures of 0.degree.C and around 200.degree.C. Such known nickel-iron alloys, however, have low strength, and it is also known to strengthen these alloys, for use in wrought form, with additions of an element such as titanium which forms an intermetallic compound precipitate when the wrought alloy is given an ageing heat treatment. However, as is also well known, the increase in strength resulting from such additions is achieved at the expense of the low coefficient of thermal expansion which tends to be undesirably increased with increase in strength and this undesirable effect would be expected to be even more marked for the alloy in cast form than for the alloy in wrought form.
It is now proposed to strengthen low expansion austenitic nickel-iron (-cobalt) alloys, for use in cast form, by the addition of carbide forming elements. Normally additions to nickel-iron alloys of carbide forming elements such as, for example, carbon, chromium, molybdenum would be expected significantly to increase the low thermal expansion coefficient attainable in nickel-iron alloys. Surprisingly it has now been found that if the nickel content, or where cobalt is also present, the sum of the nickel and cobalt contents, is maintained within specific limits, additions of carbon and vanadium, which form carbides with carbon, and optionally chromium and/or molybdenum which also form carbides with carbon, can be made which substantially increase the strength of nickel-iron (-cobalt) alloy castings with or without ageing, while at the same time allowing a low thermal expansion coefficient to be largely maintained.
It is an object of the present invention to provide nickel-iron or nickel-iron-cobalt alloys having in the cast condition a low thermal expansion coefficient and good tensile strength.
It is another object of this invention to provide a low expansion alloy which can be cast as a structural component requiring close dimensional tolerances under varying temperature conditions.
It is a further object to provide a low-expansion alloy with good strength and suitable for investment casting into complex structures, which can be cast in air.
It is still another object to provide a cast structural machine part, including rotating and reciprocal parts such as turbine blades and shafts, which operate in close proximity to other machine parts and require dimensional stability and good tensile strength over varying temperatures up to about 300.degree.C or higher, e.g., up to about 500.degree.C or 600.degree.C.
These and other objects and advantages will become apparent from the following description of the invention and the examples.